Digital magnetic tape is commonly used for long-term data storage in computer systems. The low cost-per-bit stored, long-term retention capability, and portability of magnetic tape cartridges have made magnetic tape the medium of choice for storing the large quantities of data generated by businesses such as insurance companies, banking organizations, airlines, and the like.
Tape cartridges are frequently utilized by automated tape library peripherals. Tape libraries generally handle numerous tape cartridges to store very large amounts of data. The tape libraries hold the tape cartridges in predefined positions or media slots. When content is required from a particular tape cartridge, a host system communicatively coupled to the tape library communicates with a robotics subsystem controller of the tape library. The host system may be communicatively coupled to the tape library by a number of mechanisms. For example, the host system may be communicatively coupled to the tape library via a Small Computer System Interface (SCSI) bus or a Fibre Channel fabric. The host system may communicate a command to the robotics subsystem controller to retrieve the particular tape cartridge from the respective position or media slot. In response, the robotics subsystem controller may execute various mechanical operations by robotic mechanics to retrieve the desired tape cartridge and place the desired tape cartridge into a tape drive. The host system may typically communicate with the tape drive utilizing a communication path independent of the communication path to the robotics subsystem controller. The host system may then read from or write to the tape cartridge via the tape drive.
Additionally, other communication may occur between the host system and the robotics subsystem controller. For example, the host system may query the robotics subsystem controller to determine the number of tape cartridges, tape drives, and robotics mechanisms contained in the tape library associated with the robotics subsystem. By querying the robotics subsystem controller in this manner, the host system may manage the tape library.
Although tape libraries are useful for providing a significant amount of long-term data storage capacity, tape libraries possess several disadvantages. For example, the data storage capacity may be allocated for use by several entities. Accordingly, all of the data storage capacity may be accessible by each host system or entity communicatively coupled to the tape library. To address this problem, partitioning has been utilized. Partitioning involves assigning specific tape library resources (e.g., various slots, tape drives, robotic mechanics) to discrete virtual devices. Access to the virtual devices is then restricted as desired for particular applications.
Different approaches may be utilized to partition a tape library. First, host system restrictions have been utilized to restrict access to portions of a tape library for partitioning applications. Such host restrictions are implemented by a mediating software process on a host system to enforce partition restrictions. However, this approach is problematic. Specifically, the approach is undesirable if the tape library is utilized in a storage service provider environment. In storage service provider environments, the tape library and the host systems belong to different entities (e.g., the storage service provider and the customers). Placement of software mediating processes on host systems is unattractive, because it increases the burden on the customers to make use of the storage service. Also, corporate environments impose relatively long qualification cycles for new host system software. Moreover, many customers are unwilling to allow other parties to place software on their host systems. Additionally, the software mediating process approach is typically incompatible with existing data back-up utilities, i.e., the software mediating process approach requires the use of specialized data back-up applications.
A second approach, used by Spectra Logic, provides multiple bridge devices to partition tape libraries into multiple virtual devices. Each bridge device includes a Quad Interface Processor (QIP) to implement partitioning. The QIPs are processor-based card devices that serve as the sole interface between a host system and a library. By configuring each of the QIPs via a software interface, the QIPs are capable of emulating several virtual libraries that appear to be multiple physical libraries to a host system or host systems. Each QIP typically includes two Small Computer System Interface (SCSI) buses and up to two tape drives may be coupled to each SCSI bus.
Although this architecture does provide a mechanism to implement partitioning in a manner that is somewhat transparent to host systems this architecture is problematic. Specifically, it is costly and hardware-intensive in that it requires multiple processor-based cards. Second, the use of multiple hardware bridges introduces multiple points of failure, thereby increasing the probability of that data may become inaccessible. Moreover, this hardware approach rigidly controls the addition of further resources to a given partition. There is a specific structural relationship between the bridging hardware and the addition of further resources to a partition. Accordingly, the hardware approach is not amenable to the creation of multiple smaller partitions or applicable to partitioning smaller tape libraries.